The invention concerns a process for the manufacture of a part with very high mechanical properties, formed by stamping of a strip of rolled steel sheet and more particularly hot rolled and coated with a metal or metal alloy ensuring protection of the surface and the steel.
The steel sheets intended for high temperature forming and/or heat treatment are not delivered with a coating in view of the retention of the coating during the heat treatment, as steels are generally heat treated at relatively high temperatures, far in excess of 700xc2x0 C. Indeed, zinc coating deposited on a metallic surface was considered heretofore as likely to melt, flow, foul the hot forming tools during the heat treatment at temperatures in excess of the zinc melting temperature, and degrade during quenching.
Therefore, the coating is applied on the finished part, which necessitates careful cleaning of the surfaces and hollow areas. Said cleaning requires the use of acids or bases, whose recycling and storage entail significant financial costs and risks for the operators and the environment. In addition, heat treatment must be performed under controlled atmosphere in order to prevent any steel decarburization and oxidation. Furthermore, in the heat forming process, carbon buildup damages the forming tools because of its abrasiveness, which diminishes the dimensional and aesthetic quality of the parts produced or requires frequent and costly tool repairs. Finally, in order to increase their resistance to corrosion, the parts thus obtained must undergo costly post-treatment, whose application is difficult or even impossible, in particular for parts with hollow areas. Post-coating of steels with very high mechanical properties also has the drawback of creating risks of fragilization due to hydrogen in electrogalvanizing or of alteration of the mechanical properties of the steels in bath galvanizing of previously formed parts.
The purpose of the invention is to provide users with rolled steel sheets of 0.2 mm to 4 mm in thickness, coated in particular after hot rolling, to undergo either hot or cold forming, followed by heat treatment, as well as a process for the production of parts by hot forming, using these coated rolled steel sheets, where the rise in temperature is ensured without decarburization of the steel of the sheet, without oxidation of the surface of said steel sheet, before, during and after the hot forming and/or the heat treatment.
The invention concerns a process for the manufacture of a part with very, high mechanical properties, formed by stamping of a strip of rolled steel sheet and more particularly hot rolled and coated with a metal or metal alloy ensuring protection of the surface and the steel, whereby:
the steel sheet is cut to obtain a steel sheet blank,
the steel sheet blank is stamped to obtain the part,
an alloyed intermetallic compound is applied to the surface, before or after the stamping, ensuring protection against corrosion, against steel decarburization, which intermetallic compound may provide a lubrication function,
the excess material from the steel sheet required for the stamping operation is trimmed.
In a preferred embodiment of the invention,
the steel sheet is cut to obtain a steel sheet blank,
the coated steel sheet blank is subjected to a rise in temperature in order to hot-form a part,
an alloyed intermetallic compound is thereby formed at the surface of the part, ensuring protection against corrosion, against steel decarburization, which intermetallic compound may provide a lubrication function,
the steel sheet blank is fabricated by stamping,
the stamped part is cooled to obtain such mechanical properties in the steel as high hardness and high surface hardness of the coating,
the excess material from the steel sheet required for the stamping operation is trimmed.
The other characteristics of the invention include:
the metal or metallic alloy of the coating is zinc or a zinc-based alloy of a thickness ranging from 5 xcexcm to 30 xcexcm.
the intermetallic alloy is a zinc-iron based compound or a zinc-iron-aluminum based alloy.
the coated steel sheet is subjected to a rise in temperature in excess of 700xc2x0 C. prior to the stamping and/or heat treatment.
the part obtained in particular by stamping is cooled so that it is quenched at a rate higher than the critical cooling rate.
The invention also concerns the use of a strip of rolled steel sheet and more particularly hot rolled and coated with a metal or metal alloy ensuring protection of the surface and the steel of the steel sheet in the forming by stamping, in particular hot forming of parts, such parts having high mechanical properties as regards hardness and high surface hardness as well as very good resistance to abrasion.